Medical radiography has been exploited for many years as a means for observing and diagnosing internal abnormalities in living organisms. A patient is typically exposed to X-rays which are selectively and partially absorbed by tissues and bone as they pass through the patient. The X-radiation which passes partially attenuated through, the patient-carries information about the patient internal structure. Efforts aiming to capture and display this information with the highest degree of accuracy, minimal loss of information content and minimal patient exposure constitute the basis for a large part of the research effort in radiography.
Modern medical radiography systems typically utilize one or two phosphor intensifying screens which absorb X-rays and emit longer wavelength visible or ultraviolet light, thus translating the information initially contained in the X-ray beam to information carried by longer wavelength radiation. This longer wavelength image then impinges on a photographic element which is sensitive to the actinic light emitted by the intensifying screen or screens. This photographic element comprises one or two photosensitive layers coated on either side of a support. On absorbing actinic light emitted by the intensifying screen or screens a latent image is formed in the photographic element, thus rendering the photographic element developable and allowing an image of the information originally carried by the X-ray beam to be recorded as a permanent image.
It is the aim of film/screen radiographic systems to provide an image which records the information carried by the X-ray beam traversing the patient as faithfully as possible. However, the ability of the film/screen system to render a faithful reproduction of the information carried by the X-ray beam is limited by
(a) the various physical processes which occur in the course of converting the X-ray energy to visible light and
(b) the film's limited response to the generally wide range of light intensities constituting the converted image. The conversion of X-ray energy to actinic light depends on the probability of each one of a sequence of steps as follows:
(a) the probability that the screen will absorb an incoming X-ray quantum,
(b) the probability that a phosphor particle will emit a certain number of light photons, and finally
(c) the probability that a light photon, once emitted by a phosphor particle and then being subjected to light scattering and possible absorption within the screen, will emerge at the surface of the screen facing the photosensitive element.
The fact that the X-ray absorption and X-ray to light conversion processes are statistical introduces random fluctuations in light intensity which are recorded as image noise and are commonly called "quantum noise"; light scattering also introduces noise, but, because of the increased chance that light photons emitted deep within the screen and at wide angles with respect to the screen's surface will be absorbed, light scattering limits the size of the cone of light which emerges on the screen's surface and whose apex lies at the point where an X-ray quantum has been absorbed within the screen. Thus, light scattering coupled with internal light absorption actually improves the screen's ability to faithfully reproduce the information carried by the X-ray beam in that it improves the screen's ability to resolve fine details, i.e., by improving the screen's resolution as determined by its Modulation Transfer Function (MTF).
The ability of the film to faithfully record the converted image emerging from the screen is limited by its dynamic range, i.e., by the ability of the film to produce an observable change in developed optical density for the entire range of light intensities (exposures) projected by the image emanating from the screen. In general, the film is only able to record, i.e., to produce observable changes in developed optical density, for a limited range of exposures and this is characterized by the film's contrast curve, a plot of dD/d(log E) vs. log E (where D is the developed optical density and E is the exposure), which is familiar in the art. The range of exposures over which there is adequate contrast is defined by a lower and an upper exposure limit. The lower exposure limit is that which will produce a density which is detectable above the base plus fog. The upper exposure limit is defined as that which will produce an output density which is detectable below the maximum density of the film. If the differences in X-ray absorption by the tissues of the subject are small, such as in mammography where very little difference in the density of the tissues is observed, the subject contrast is considered low and it is necessary to use a film whose contrast is high in order to render the small differences in absorption distinguishable in the resulting radiograph. Conversely, if the subject matter being examined provides large differences in X-ray absorption (e.g. chest radiography or angiography where the difference in density of bone and soft tissue is large) the contrast of the film may be lower because the subject contrast is already adequately high.
Lastly, in those systems employing two screens to expose photosensitive elements having a photosensitive layer coated on both sides of a support, i.e., so-called double side coated films, there is an additional source of image degradation called "print-through" or "cross-over" in which light emanating from the front screen (i.e., the screen nearest to the incoming X-ray beam) penetrates through the transparent support to the photosensitive layer on the back side of the support and vice versa. This crossing over of light from front to back and vice versa causes light to spread as it passes through the transparent support and thus degrades the ability of the recording system to record fine details, i.e., the systems resolution as measured by its MTF.
FIG. 1 shows the combined effects of light scattering coupled with internal light absorption and of print-through on the resolution of the two-screen/double side coated film system. A phosphor particle 1, within the upper screen 2, absorbs an X-ray quantum 3, and emits light 4. While some of the emitted light, particularly light emitted at a large angle with respect to the screen's surface is absorbed within the screen, a fraction of the emitted light emerges, forming a light cone whose diameter at the screen's surface is much larger than the point from which the light originally emanated, thereby resulting in a loss of resolution. A fraction of this emerging light is absorbed by the photosensitive layer 5, adjacent to the emitting screen; however, another, smaller fraction of the emitted light penetrates through the first layer 5, the transparent support 6, and is recorded by the photosensitive layer 7, on the other side of the support. As the light traverses the support 6, it is subject to further spreading, thus further degrading the system's MTF. For clarity, decrease in MTF emanating from the optional lower screen 8, is not shown.
Improvements in the photosensitive element have been provided wherein a dyed layer is interposed between at least one photosensitive layer and the support as described, e.g., in Diehl and Factor, U.S. Pat. No. 4,950,586. The dyed underlayer absorbs the light which would otherwise penetrate to the photosensitive layer on the opposite side. This method is disadvantageous in that the additional dyed underlayers increase manufacture cost and may impart an objectionable color to the photographic element by leaving a dye residue after processing. Moreover, the screens still limit the systems resolution as described above, since, even with "zero" print-through, the best image which can be captured by the film is that which is emitted by the screen.
One method to improve the film's dynamic range well known to the art and referenced in Formulating X-ray Technics, 5th ed., Cahoon, Duke University Press, 1961, pg. 11, is to use a thin front screen to "balance" the front and back screen's X-ray absorption. Thus, for instance, if the front screen absorbs 30 of 100 incoming X-ray quanta (30%) then, to make the back screen's absorption equal to that of front screen, it must absorb the remaining 70 photons [i.e., 100.times.(30/70)=42.8%]. Thus, since the back screen must then absorb a larger percentage of X-ray quanta, the back screen must have a higher phosphor coating weight than the front screen. However, since the MTF of the screen decreases with increasing phosphor coating weight, this method tends to further decrease the MTF of the back screen and thereby the MTF of the image recorded by the film.
In a recent development disclosed by Bunch and Dickerson, EP 384643A, a "zero" print-through film, using a dyed underlayer and having emulsions with widely differing contrast curves is used in combination with a thin/thick screen pair to widen the dynamic range of the system. The above mentioned limitations on resolution and disadvantages of using a dyed underlayer also apply to the teachings of Bunch and Dickerson.
In general, it is known to the art that the signal recorded by the film/screen system can be characterized by means of the Contrast Transfer Function (CTF) as described in Dainty and Shaw, Imaging Science, Academic Press, London-New York-San Francisco 1974, p.234ff. This function is defined by: EQU CTF(f,E)=MTF(f).times..gamma.(E)
where f is the spatial frequency of a test object, E is the exposure and .gamma.(E) is the contrast function which is defined as: EQU .gamma.(E)=dD/d(logE)
wherein D is optical density and E is exposure. This function shows how the system contrast decreases with increasing spatial frequency. The slower the decrease in CTF with increasing spatial frequency and the wider the range of exposures over which the contrast is significantly higher than zero, the greater is the ability of the system to record a wide range of information.
Contrast (.gamma.(E)) curves are provided in FIG. 5 wherein Curve A represents a conventional high contrast film, Curve B represents a conventional latitude film and Curve C represents a very wide latitude film. In accordance with the definition of CTF(f,E) a contrast film with a higher .gamma.(E) exhibits a higher CTF(f,E) than the corresponding latitude film at the same relative exposure and system MTF(f). To achieve this increased CTF(f,E), or image resolution, the useable exposure range is decreased versus the latitude films of Curves B and C as indicated in FIG. 5.
It is the aim of the present invention to circumvent the above mentioned deficiencies in the art by improving the useable exposure range of diagnostic radiographic images without loss of image quality.